EXPLORATION, RECOVERY, AND TRANSPORTATION 81
movement, or it may be brought about through the use of a pump
jack, which is connected to a central power source by means of
pull rods. Electrically powered centrifugal pumps and submersible
pumps — both pump and motor are in the well at the bottom of the
tubing — have proven their production capabilities in numerous
applications.
There are also secondary oil recovery operations that involve
the injection of water or gas into the reservoir. When water is used
the process is called a waterflood; with gas, the process is called a
gasflood. Separate wells are usually used for injection and production. The injected fluids maintain reservoir pressure or re-pressure
the reservoir after primary depletion and displace a portion of the
remaining crude oil to production wells. In fact, the first method
recommended for improving the recovery of oil was probably the
re-injection of natural gas, and there are indications that gas injection was utilized for this purpose before 1900 (Craft and Hawkins,
1959; Frick, 1962). These early practices were implemented to
increase the immediate productivity and are therefore classified
as pressure maintenance projects. Recent gas injection techniques
have been devised to increase the ultimate recovery, thus qualifying as secondary recovery projects.
In secondary recovery, the injected fluid must dislodge the oil
and propel it toward the production wells. Reservoir energy must
also be increased to displace the oil. Using techniques such as gas
and water injection, there is no change in the state of oil. Similarly,
there is no change in the state of the oil during miscible fluid displacement technologies. The analogy that might be used is that of
a swimmer in which there is no change to the natural state of the
human body.
Thus, the success of secondary recovery processes depends on
the mechanism by which the injected fluid displaces the oil (displacement efficiency) and on the volume of the reservoir that the
injected fluid enters (conformance or sweep efficiency). In most
proposed secondary projects, water does both these things more
effectively than gas. It must be decided if the use of gas offers any
economic advantages because of availability and relative ease
of injection. In reservoirs with high permeability and high vertical span, the injection of gas may result in high recovery factors
as a result of gravity segregation, as described in a later section.
However, if the reservoir lacks either adequate vertical permeability or the possibility for gravity segregation, a frontal drive
movement, or it may be brought about through the use of a pump
jack, which is connected to a central power source by means of
pull rods. Electrically powered centrifugal pumps and submersible
pumps — both pump and motor are in the well at the bottom of the
tubing — have proven their production capabilities in numerous
applications.
There are also secondary oil recovery operations that involve
the injection of water or gas into the reservoir. When water is used
the process is called a waterflood; with gas, the process is called a
gasflood. Separate wells are usually used for injection and production. The injected fluids maintain reservoir pressure or re-pressure
the reservoir after primary depletion and displace a portion of the
remaining crude oil to production wells. In fact, the first method
recommended for improving the recovery of oil was probably the
re-injection of natural gas, and there are indications that gas injection was utilized for this purpose before 1900 (Craft and Hawkins,
1959; Frick, 1962). These early practices were implemented to
increase the immediate productivity and are therefore classified
as pressure maintenance projects. Recent gas injection techniques
have been devised to increase the ultimate recovery, thus qualifying as secondary recovery projects.
In secondary recovery, the injected fluid must dislodge the oil
and propel it toward the production wells. Reservoir energy must
also be increased to displace the oil. Using techniques such as gas
and water injection, there is no change in the state of oil. Similarly,
there is no change in the state of the oil during miscible fluid displacement technologies. The analogy that might be used is that of
a swimmer in which there is no change to the natural state of the
human body.
Thus, the success of secondary recovery processes depends on
the mechanism by which the injected fluid displaces the oil (displacement efficiency) and on the volume of the reservoir that the
injected fluid enters (conformance or sweep efficiency). In most
proposed secondary projects, water does both these things more
effectively than gas. It must be decided if the use of gas offers any
economic advantages because of availability and relative ease
of injection. In reservoirs with high permeability and high vertical span, the injection of gas may result in high recovery factors
as a result of gravity segregation, as described in a later section.
However, if the reservoir lacks either adequate vertical permeability or the possibility for gravity segregation, a frontal drive